Over the last decade, silicon-nitride Si3N4 balls have become an important component of advanced bearings used in a wide range of applications. The greatest commercial success for Si3N4 balls has been their use in hybrid bearings that combine the ceramic balls with steel races and that are known as silicon-nitride hybrid bearings. Compared to the steel balls, which the silicon-nitride balls replace, the ceramic balls are harder and less dense and offer higher compressive strength, better corrosion resistance, elevated operating temperature, and reduced lubrication requirements. These benefits make the hybrid bearings ideal for severe high-speed applications such as machine tool spindles, high speed dental drills, vacuum turbomolecular pumps, and the liquid-oxygen turbomolecular pumps used in the space shuttle main engines. Large diameter ceramic balls recently became the leading technology for hip replacements. The FDA requires fracture toughness testing of a substituted rectilinear specimen. Hence, a rigorous fracture toughness test that can be used by the FDA and orthopedic manufacturers is desirable for hip ball joints as well.
For exemplar future use in the space industry, the hybrid bearings have been proposed for the improved momentum control wheels and flywheels for satellites. Importantly, the hybrid bearings have recently been used in roller blades, an application where the bearings represent a mass marketing opportunity for lightweight rugged bearings. The roller blade market, as well as other commercial applications, provides recreational users and athletes with cost-effective high technology long-lasting bearings with improved performance in high volumes that would lower the price of the hybrid bearings for all applications with increasing overall sales. For machine tool spindles, the market for hybrid bearings was at $35 million in 2000 and is projected to reach $150 million by 2005, and hence there is wide spread usage. The overall sales of hybrid bearing should reach several hundred million by 2010. Hence, there is a significant need for high-volume hybrid bearings subject to repeatable and accurate manufacturing requirements.
Ceramic balls have significant drawbacks and limitations. Like all ceramics, the silicon-nitride balls have a low tensile strength, which is a fundamental material property. Therefore, under applied tension, the balls are prone to crack either at a preexisting manufacturing flaw or at a flaw that develops during service and usage. A closely related fundamental material property is the fracture toughness, which indicates the susceptibility to fracture of the ceramic material. Low fracture toughness is most important factor indicating the ruggedness and usefulness of all ceramics in general as well as the silicon-nitride balls, in particular. Fortunately, highly engineered ceramics have been developed whose fracture toughness can be significantly increased through processing that controls microstructures. Precise manufacturing can control the size and number of preexisting flaws. To produce tougher ceramics is therefore a two-fold task. First, a microstructure is selected that is intrinsically tougher, which reduces the severity of any flaws. Second, the preexisting flaws are eliminated, which ameliorates the low fracture toughness.
In the specific case of silicon-nitride balls, manufacturing processing has been developed that provides, for example, a two-phase microstructure of alpha and beta silicon nitride where the minor second phase is a blocky shape in a matrix of the major phase. As a micromechanism, this microstructure promotes crack deflection and blunting, which raises the intrinsic fracture toughness. In addition, the ceramic balls are manufactured from a starting powder through hot-isostatic pressing that is followed by grinding and lapping to provide precise spherical shapes. Accurate control of the hot-isostatic pressing eliminates sintering voids and inclusions that are potential preexisting flaws leading to potential fracture and failure of the balls. Precise control of the grinding and lapping eliminates surface cracks. Inspection and nondestructive evaluations are also used to screen balls with preexisting flaws from usage especially in critical applications.
Hence, it is highly desirable to have a manufacturing test that measures the fracture toughness of mass produced silicon-nitride balls. Ideally, the test should be simple and robust enough to b used for quality control by manufacturers and for qualification by contractors installing the balls in critical applications. As an added benefit, a robust quality control manufacturing screening test offers the ability to specify fracture toughness, which is a basic material property, as a purchasing requirement. The manufacturing screening fracture toughness test can also be incorporated into statistical process control on a manufacturing factory floor to reduce cost, improve quality, and to evaluate independently the success of the inspection and nondestructive evaluations. In addition, the manufacturing screening fracture toughness test can be used to research how changes in materials processing control fracture toughness so as to provide testing feedback between various manufacturing processes and mechanical behavior of the ceramics.
There are basically two different classes of tests for fracture toughness of brittle materials such as ceramics. The first class is a direct measurement in which the applied stress state at which a crack grows is measured. The second class is based upon indentation techniques and is indirect because the applied stress state is inferred from semiquantitative estimates of residual stress based upon an indirect dimensional argument. In the direct measurement, a starting crack or flaw of known size and shape is placed in the test specimen. The geometry of the starting crack directly gives the stress concentration of the crack based upon either an analytical solution or a finite element solution. The specimen is then loaded so that the crack is under tension. Both the observed applied load at which the crack grows and the calculated stress concentration of the crack are combined to give directly the ceramic fracture toughness of the ceramic. For a standard direct test of fracture toughness, the fracture toughness is defined by the observed load at which the crack grows and the geometry of the starter crack, which provides the stress concentration at the starter crack.
Examples of direct measurements include the chevron notch, bridge indentation, double cantilever beam and Tattersall-Tappin tests. Unfortunately, these tests all use a rectilinear or cylindrical specimen that typically has a largest linear dimension that is on the order of several centimeters. In contrast, ceramic balls used in even the largest capacity hybrid bearings have a diameter of much less than 1.5 cm, which makes it virtually impossible to fabricate specimens for any of these standard fracture tests due to the spherical shape of the balls and the limited volume of the balls.
As an alternative, the indentation test is currently used in industry to measure the fracture toughness of the ceramic balls. In this test, either a Vickers or Knoop indenter is used to make a pyramidal indentation in the surface of the ceramic. Indentation tests are indirect and semiquantitative. At a sufficiently high applied indentation load, cracks can grow from the corners of the indentation as the indentation is loaded. The length of the produced cracks and the maximum applied load are used to calculate the fracture toughness of the ceramic. In contrast to the direct tests, crack growth is not caused directly by the applied load but is instead caused physically by accommodation of displaced material aft r the crack is unloaded. The indentation test is, however, not rigorously quantitative. The calculation is based upon an estimate of the residual tensile stresses caused by the indentation. The estimate is not based upon direct measurements but is instead based upon a simple and crude dimensional analysis. The calculation also requires a constant that characterizes the volumetric change during indentation due to plastic flow of the indented ceramic. A value for the constant is selected that only averages the observed volumetric change of all the different ceramics ever measured. Ideally, the constant should be calibrated through an independent measurement of the toughness of the ceramic.
Disadvantageously, there is no current known test that can independently measure the fracture toughness of the ceramic balls. The existing fracture toughness test methods are disadvantageously indirect and imprecise because these test methods rely upon a semiquantitative estimate of residual stress. In addition, these have an additional source of imprecision because the residual stress estimate should ideally be calibrated through an independent direct measurement of fracture toughness. Disadvantageously, the existing test methods that directly measure the fracture toughness have only been applied to rectilinear specimens. These and other disadvantages are solved or reduced using the invention.